This invention is generally directed to processes for treating amorphous silicon, and more specifically, the present invention is directed to a simple direct, economically attractive process for restoring amorphous silicon photoconductive substances with fluorine containing compositions. In one embodiment, the present invention is directed to a process for treating virgin amorphous silicon photoconductive substances with fluorine containing compositions, such as hydrofluoric acid, thereby increasing the charge acceptance of these substances. Also, in accordance with the process of the present invention the charge acceptance of amorphous silicon photoconductors can be increased subsequent to their use in an imaging apparatus. Moreover, in accordance with the process of the present invention amorphous silicon containing undesirable invisible latent scratches on the surface thereof can be treated with fluorine containing substances for the purpose of eliminating the print out of these scratches. Further, in accordance with the process of the present invention image defects which appear as white spots obtained with virgin electrophotographic amorphous silicon devices can be eliminated. Additionally, by treating amorphous silicon in accordance with the process described hereinafter aging of stored photoconductive drums, which translates into loss of image resolution, can be prevented.
In one important embodiment of the present invention, the process comprises restoring hydrogenated or halogenated amorphous silicon imaging members by contacting these members with vapors of hydrogen fluoride for a period of from about 1 minute to about 240 minutes thereby resulting in a member that when reincorporated into electrophotographic imaging devices enables the achievement of images of increased resolution with substantially no white spots as compared to the member prior to treatment, which is substantially unusable.
Amorphous silicon photoconductors treated in accordance with the process of the present invention are useful as photoconductive imaging members in an electrophotographic imaging apparatus wherein, for example, electrostatic latent images formed on the surface thereof are developed with toner particles, transferred to a suitable substrate such as paper, and subsequently optionally permanently affixed thereto by heat for example.
Electrophotographic imaging systems, particularly xerographic imaging systems, are well known, and are extensively described in the prior art. In these systems, generally a photoresponsive or photoconductor material is selected for forming the latent electrostatic image thereon. This photoreceptor is generally comprised of a conductive substrate containing on its surface a layer of photoconductive material, and in many instances a thin barrier layer is situated between the substrate and the photoconductive layer to prevent charge injection from the substrate as injection would adversely effect the quality of the resulting image. Examples of known useful photoconductive materials include amorphous selenium, alloys of selenium such as selenium tellurium, selenium arsenic, and the like. Additionally, there can be selected as the photoresponsive imaging member various organic photoconductive materials, including, for example, complexes of trinitrofluorenone and polyvinyl carbazole. Recently, there have been disclosed layered organic photoresponsive devices containing separate charge transport and photogenerating layers. Examples of charge transport layers include various diamines, while useful photogenerating compositions include trigonal selenium, metal and metal-free phthalocyanines, vanadyl phthalocyanines, and the like.
Also known are amorphous silicon photoconductors, reference for example U.S. Pat. No. 4,265,991. There is disclosed in this patent an electrophotographic photosensitive member containing a substrate, a barrier layer, and a photoconductive overlayer of amorphous silicon containing 10 to 40 atomic percent of hydrogen, and having a thickness of 5 to 80 microns. Additionally, this patent describes several processes for preparing amorphous silicon. In one process, there is prepared, according to the teachings of this patent, an electrophotographic sensitive member by heating the member contained in a chamber to a temperature of 50.degree. C. to 350.degree. C., introducing a gas containing silicon and hydrogen atoms into the chamber, causing an electrical discharge in the space of the chamber, followed by depositing amorphous silicon on an electrophotographic substrate at a rate of 0.5 to 100 Angstroms per second. The charge acceptance of these devices is found to be limited by the surface conditions. Further, in many instances as a result of corona interaction with the surfaces of the devices prepared in accordance with the process of this patient, the surface conductivity undesirably increases resulting in a loss of image resolution, and a decrease in image density within less than about 1,000 imaging cycles. Accordingly, while the amorphous silicon photosensitive devices of the '991 patent are useful, their selection as a commercial device for a number of imaging cycles is not readily achievable.
Disclosed in U.S. Pat. No. 4,357,179, issued Nov. 2, 1982, is a method for preparing devices containing high density amorphous silicon or germanium wherein with respect to the amorphous silicon, p-doping or b-doping can be affected with dopants such as phosphorous or boron, reference the disclosure in column 5, beginning at line 44, and specifically at lines 55 to 65.
Also, in U.S. Pat. No. 4,237,150, issued Dec. 2, 1980, there is disclosed a method for preparing hydrogenated amorphous silicon wherein ammonia is introduced into a reaction chamber with a silane gas for the purpose of enhancing the photoconductivity of the resulting hydrogenated amorphous silicon films, reference the disclosure in column 1, beginning at around line 20.
Further, U.S. Pat. No. 4,237,151, issued Dec. 2, 1980, discloses the preparation of hydrogenated amorphous silicon substances by thermally decomposing silane or other gases at elevated temperatures and under specific vacuum conditions wherein a gaseous mixture of atomic hydrogen and atomic silicon result, followed by depositing this mixture onto a substrate situated outside a heated tungsten tube wherein a film of hydrogenated amorphous silicon is formed on the substrate. In column 4, beginning at line 58, it is indicated that conventional doping gases can be added to the silane if desired.
Furthermore, U.S. Pat. No. 4,356,246, issued Oct. 26, 1982, describes a specific noncrystalline silicon powder having excellent photoconductivity comprised of silicon and hydrogen, which powder exhibits specific characteristics such as an infrared absorption spectrum characterized by absorption peeks centered about certain areas, reference the Abstract of the Disclosure. There is disclosed in column 5, beginning at line 41, that in addition to hydrogen other elements such as oxygen, fluorine, chlorine, bromine, iodine, phosphorous, boron, solely or in the form of a combination, may be contained in the amorphous silicon particles for the purpose of controlling the electrical conductivity thereof.
Additionally, U.S. Pat. No. 4,361,638, issed Nov. 30, 1982, discloses a light sensitive electrophotographic element including a photoconductive layer comprised of amorphous silicon and a carbon based material doped with hydrogen and fluorine, reference the disclosure in column 4, beginning at line 3.
Additionally, U.S. Pat. No. 4,365,013, issued Dec. 21, 1982, discloses an amorphous silicon layer, which apparently can be rendered highly photoconductive by doping with hydrogen, or by doping with impurities, reference the disclosure in column 2, beginning at line 46. Doping materials disclosed include halogens such as fluorine, chlorine, bromine and iodine, reference the disclosure in column 2, beginning at line 57.
Other references of interest include U.S. Pat. Nos. 4,342,044; 4,394,426; 4,468,443 and 4,490,208. In the '044 patent, reference column 4, beginning at line 54, and particularly at line 65, there is disclosed that the nonoptimum spectral response of the prior art amorphous silicon photoresponsive devices is overcome by adding one or more band gap adjusting elements to the amorphous photoresponsive alloy. More specifically, as stated at line 65, "The amorphous alloy incorporates at least one density of states reducing element, fluorine. The compensating or altering element, fluorine or elements, can be added during deposition or thereafter.". Also, disclosed in the '208 patent are methods of obtaining thin films of silicon by doping a p or n type thin film of silicon with an impurity element under a plasma discharge of gas of at least one element selected from among fluorine, chlorine, bromine, iodine, and hydrogen, reference column 1, lines 51 to 63. In the '426 patent, there is disclosed a photoconductive member containing amorphous silicon atoms as a matrix, hydrogen or halogen atoms; and as an intermediate layer an amorphous material containing, for example, silicon atoms and nitrogen atoms.
Additionally, described in U.S. Pat. No. 4,634,647, the disclosure of which is totally incorporated herein by reference, are processes for preparing amorphous silicon, which is useful for incorporation into an electrophotographic imaging apparatus, wherein images of high resolution can be obtained for a number of imaging cycles. This process involves simultaneously treating amorphous silicon with dopants such as boron and phosphorous.
Recently, substantial interest has been directed to obtaining amorphous silicon photoreceptor materials since they possess a number of advantages in comparison to, for example, amorphous selenium based materials in that amorphous silicon is of extreme hardness and will not crystallize over extended time periods even at temperatures as high as several hundred degrees Centigrade. Additionally, amorphous silicon photoreceptor materials have excellent photoelectric properties, high absorption coefficients through the visible region, and are relatively low in useful life cost in comparison to selenium photoconductors. Moreover, amorphous silicon photoreceptors are capable of ambipolarity as they can be xerographically charged, and discharged either positively or negatively in various imaging systems. Furthermore, amorphous silicon can be modified by adding various dopants thereto such as boron and phosphorous, enabling this material to function as a p or n type semiconductor device; and amorphous silicon may be alloyed with other materials including germanium and tin for the purpose of providing a material which will be photosensitive in the infrared region of the spectrum. Moreover, amorphous silicon materials are inert and nontoxic rendering them highly desirable as a photoconductive imaging member.
While processes are known for preparing amorphous silicon, in most instances these processes result in members which, after repeated usage in electrophotographic imaging devices, have decreased charge acceptance causing the resulting images to be of poor density. While it is not desired to be limited by theory, it is believed that as a result of a continuous mechanical interaction of developer carrier beads with the surface of the amorphous silicon, the silicon-hydrogen, and/or silicon-silicon bonds are broken, and in the presence of water vapor are reformed into silicon-hydroxy type bonds. The charge acceptance of these devices, which is limited by the surface conditions, decreases as a result of this mechanical interaction causing an undesirable reduction in the density of the images produced. Further, the mechanical damage caused by the developer beads during the development step lowers the charge acceptance of the entire amorphous silicon photoresponsive device or drum, and causes a corresponding decrease in the density of the total image.
Another type of mechanical damage observable with many hydrogenated or halogenated amorphous silicon imaging members is confined to small isolated areas of the surface, this damage generally being referred to as scratches. These scratches are caused, for example, during xerographic imaging processes wherein the amorphous silicon photoconductor is subjected to cleaning with wiper blades. Furthermore, interaction of the amorphous silicon with isolated carrier particles contained in the developer mixture can cause scratches. Additionally, these scratches can be generated during the handling of the amorphous silicon drum while it is being manufactured, and during the positioning of this drum within the machine involved. These latent scratches, which are not visible, damage the surface of the amorphous silicon causing a reduction in the charge acceptance thereof, and a print out of the scratches. Thus, subsequent to a minimum number of imaging cycles mechanical interaction of amorphous silicon member with carrier beads causes a reduction in image density wherein scratches print out. Eventually, the amorphous silicon photoreceptor, which was initially selected for its durability, may no longer be useful, and thus is discarded.
Additionally, it is believed that the degradation of the electrophotographic performance of amorphous silicon is caused by the sensitivity of the silicon imaging device to chemical alterations by exposure to a corona atmosphere, especially at high humidities. These sensitivities create fundamental limitations for the practical use of devices wherein the exposed surface contains substantially amorphous silicon. While this problem can be minimized by encapsulating the silicon with a chemically passive hard overcoating of amorphous silicon nitride, amorphous silicon carbide, or amorphous carbon, such devices when incorporated into xerographic imaging systems can result in image blurring and very rapid image deletion in a few imaging cycles, typically less than about ten. Furthermore, in these overcoated devices poor image quality with cycling is caused by an increase in the surface conductivity of the underlying amorphous silicon layer, rather than to abrasion or chemical interaction as occurs with amorphous silicon containing no protective overcoating layer. This conductivity increase is induced by the electric field existing at the surface of the overcoated device, similar to the effect resulting from the field effect in well known metal-insulator-semiconductor devices, causing a lateral spreading of the photogenerated charges in the fringe electric fields associated with line or edge images projected on the photoreceptor surface thereby resulting in undesirable image blurring and image deletion.
Moreover, certain freshly prepared amorphous silicon imaging members possess small defect regions referred to as white spots that accept no charge. White spots, which are generated during duplication of originals with dark solid areas, are most likely caused by structural defects in the amorphous silicon. Furthermore, it is known that amorphous silicon photoreceptors undesirably age with storage, thus although fair resolution of the images obtained with such devices is observed within a short period subsequent to fabrication, the devices when stored for a few months generate images with a loss of resolution. It is believed that these changes are caused by chemical modifications of the amorphous silicon surface as a result, for example, of the interaction of the amorphous silicon with its ambient surroundings.
Accordingly, there continues to be a need for obtaining amorphous silicon imaging members which can be repeatedly used in a number of imaging cycles. Additionally, there continues to be a need for processes for treating amorphous silicon photoconductive materials for the purpose of increasing charge acceptance and resolution thereof, and eliminating surface scratches and white spots therefrom. Moreover, there continues to be a need for restored amorphous silicon photoconductive materials thereby resulting in the elimination of surface scratches and an increase in charge acceptance for these materials. Further, there continues to be a need for processes wherein virgin unused amorphous silicon photoconductive substances can be treated primarily for the purpose of increasing the charge acceptance thereof. Also, there continues to be a need for direct economical processes for restoring amorphous silicon imaging members, which subsequent to restoration can be selected for incorporation into an electrophotographic imaging system, and wherein there results increased charge acceptance allowing the restored member to be useful for a substantial number of imaging cycles without causing a degradation in image quality, and specifically without resulting in images of low density and poor resolution. Furthermore, there continues to be a need for processes to refurbish or rejuvenate amorphous silicon drums that show loss of density and scratch print out subsequent to use in an imaging apparatus.